This invention relates generally to methods and apparatus for eddy current inspection of metallic posts contoured in one direction, such as turbine engine dovetail posts.
At least one known gas turbine rotor assemblies include a rotor wheel to which a plurality of blades are coupled. The blades extend radially outward from a platform that extends between an airfoil portion of the blade and a dovetail portion of the blade. The dovetail portion of the blade has at least one pair of dovetail tangs that couples each rotor blade to a complementary dovetail slot in an outer rim of the rotor wheel. Each of these slots is formed at an angle, called a skew angle, relative to the turbine center line.
Dovetail slots in the outer rim are sized to receive the dovetail tangs of the dovetail portion of the blade. The dovetail slot has at least one thick corner and one acute corner. The acute corners in a dovetail slot represent a natural geometric stress concentration, which is accentuated by the dovetail slot skew angle and the operational duty of the equipment. If cracks initiate during service, they may become susceptible to high cycle fatigue. Over time, continued operation with dovetail slots may result in blade release. If cracks are found near these locations through normal rotor maintenance, the affected rotors may be either retired from service or repaired.
At least one known method for inspecting gas turbine dovetail slots uses a single hand-held eddy current probe. Inspectors move a hand held probe over an area of a dovetial slot to be inspected. During manual probing, a technician watches an oscilloscope to look for defect signals. Use of this type of probe makes it difficult to provide complete coverage of the dovetail slot and slot bottom during an inspection and to record collected data for further evaluation or comparison. Also, it is difficult for a small, single coil to traverse the geometry of the dovetail, thereby making it necessary to complete multiple passes to test a single dovetail as well as resulting in a relatively low probability of defect detection. Because of the large amount of time required to perform inspections using a single hand-held probe, it is difficult and expensive to perform a complete inspection.
U.S. Pat. No. 5,659,248 to Hedengren et al. describes an eddy current surface measurement array structure for complete coverage of an underlying inspection surface without requiring mechanical scanning. A three dimensional array of eddy current sense elements is organized as a plurality of layers of two-dimensional sub-arrays. The sub-arrays, although in different layers, are essentially identical in configuration and are staggered such that the sense elements of one layer provide at least partial coverage of portions of the inspection surface not covered by the sense elements of another layer. The sense elements are disposed in a layered flexible structure fabricated employing high density interconnection fabrication techniques or other photolithographic techniques. Static (electronic) scanning is employed, by individual layer and by row and column within each layer, to form a two-dimensional image of the inspection surface.
U.S. Pat. No. 6,545,467 B1 to Batzinger et al. discloses that eddy current inspection of a contoured workpiece is performed by forming a backing piece of flexible, resiliently yieldable material with a contoured exterior surface conforming in shape to the workpiece contoured surface. The backing piece is preferably cast in place so as to conform to the workpiece contoured surface. A flexible eddy current array probe is attached to the contoured exterior surface of the backing piece such that the probe faces the contoured surface of the workpiece to be inspected when the backing piece is disposed adjacent to the workpiece. The backing piece is then expanded volumetrically by inserting at least one shim into a slot in the backing piece to provide sufficient contact pressure between the probe and the workpiece contoured surface to enable the inspection of the workpiece contoured surface to be performed.
Although the apparatus and methods of Hedengren et al. and Batzinger et al. are effective, they are relatively large and expensive to produce and require special shimming to maintain contact with a surface.